The question selection setting is widely used in many recent studies [1]}, [2]}, [3]}. But such a setting is only valid for task-specific dialog scenarios, such as the movie-on-demand dialog system described in Section REF , where all of the possible clarifying questions can be derived from the schema of the movie-entity KB. For open-domain search tasks, however, a pre-defined question bank is often not available, and the clarifying questions need to be generated on the fly, as we will discuss next.
